立方氮化硼薄膜生长过程中的界面控制

在立方氮化硼薄膜气相生长过程中生成的无定形初期层和乱层结构氮化硼中间层，一直是阻碍立方氮化硼薄膜外延生长的主要原因.系统地分析了硅衬底预处理对立方氮化硼薄膜中无定形初期层成分的影响，发现在等离子体化学气相生长法制备薄膜时，硅衬底上形成无定形初期层的可能原因有氧的存在、离子轰击以及高温下硅的氮化物的形成.在H₂气氛中1200K热处理硅衬底可以有效地减少真空室中残留氧浓度，除去硅表面的自然氧化层，保持硅衬底表面晶体结构.控制衬底温度不超过900 K，就能防止硅的氮化物的形成，成功地除去无 关键词： 立方氮化硼薄膜 等离子体化学气相生长 界面 电子显微镜     

近邻层对相邻层界面能的影响

本文采用密度泛函理论比较了三层异质结(石墨烯/石墨烯/石墨烯,石墨烯/石墨烯/氮化硼和氮化硼/石墨烯/氮化硼)和双层异质结(石墨烯/石墨烯,石墨烯/氮化硼)的结合能和广义堆垛能的差异,以研究近邻层的影响. 由于近邻层的影响,相邻层结合能会有从-2.3%到22.55%的变化,但层间距的变化很小. 此外近邻层也会影响相邻层的广义堆垛能,变化值从-2%到10%,具体的变化值依赖于相邻层的性质.     

近邻层对相邻层界面能的影响（英文）

本文采用密度泛函理论比较了三层异质结(石墨烯/石墨烯/石墨烯,石墨烯/石墨烯/氮化硼和氮化硼/石墨烯/氮化硼)和双层异质结(石墨烯/石墨烯,石墨烯/氮化硼)的结合能和广义堆垛能的差异,以研究近邻层的影响.由于近邻层的影响,相邻层结合能会有从-2.3%到22.55%的变化,但层间距的变化很小.此外近邻层也会影响相邻层的广义堆垛能,变化值从-2%到10%,具体的变化值依赖于相邻层的性质.     

石墨烯-六方氮化硼面内异质结构的扫描隧道显微学研究

石墨烯-六方氮化硼面内异质结构因可调控石墨烯的能带结构而受到广泛关注. 本文介绍了在超高真空体系内, 利用两步生长法在两类对石墨烯分别有强和弱电子掺杂的基底, 即Rh(111)和Ir(111)上制备石墨烯-六方氮化硼单原子层异质结构. 通过扫描隧道显微镜及扫描隧道谱对这两种材料的形貌和电子结构进行研究发现: 石墨烯和六方氮化硼倾向于拼接生长形成单层的异质结构, 而非形成各自分立的畴区; 在拼接边界处, 石墨烯和六方氮化硼原子结构连续无缺陷; 拼接边界多为锯齿形型, 该实验结果与密度泛函理论计算结果相符合; 拼接界面处的石墨烯和六方氮化硼分别具有各自本征的电子结构, 六方氮化硼对石墨烯未产生电子掺杂效应.     

脉冲直流偏压增强的高质量立方氮化硼薄膜的合成

采用磁增强活性反应离子镀系统成功地合成了立方氮化硼薄膜.通过给基片施加脉冲直流偏压以代替传统的射频偏压,增强了立方氮化硼的成膜稳定性,研究了基片的直流脉冲偏压、等离子体放电电流、通入气体流量比(Ar/N₂)和基片温度沉积参数对立方氮化硼薄膜形成的影响规律.结果表明:随着基片负偏压和放电电流的增大,薄膜中立方氮化硼的纯度提高,当基片负偏压为155V,放电电流为15A时,可获得几乎单相的立方氮化硼薄膜.基片温度为500℃和Ar/N₂流量比为10时,最有利于立方氮化硼 关键词： 立方氮化硼 活性反应离子镀 脉冲偏压     

无序双层六角氮化硼量子薄膜的电子性质

基于安德森紧束缚模型,本文研究了无序双层六角氮化硼量子薄膜的电子性质. 数值计算结果表明在双层都无序掺杂的情况下,六角氮化硼量子薄膜的电子是局域的, 其表现为绝缘体性质;而对于单层掺杂(无论是氮原子还是硼原子)的双层六角氮化硼量子薄膜, 在能谱的带尾出现了持续的迁移率边.这就说明在单层掺杂的双层六角氮化硼量子薄膜中产生了 金属绝缘体转变.这一结果证实了有序-无序分区掺杂的理论模型,为理解及调控双层六角氮化硼量子薄膜 的电子性质提供了有益的理论指导.     

压力对PCBN复合材料烧结性质的影响及烧结机理研究北大核心CSCD

在高温高压条件下,通过硬质合金基体的高压原位熔渗法,研究了合成压力对聚晶立方氮化硼(PCBN)复合材料致密度、颗粒形貌及耐磨性的影响。测试结果显示:随着合成压力的提高,PCBN复合材料的致密度和耐磨性均有一定程度的提升。利用扫描电子显微镜和能谱仪对碳化钨合金基体与立方氮化硼的结合界面进行了测试分析。测试结果表明:在高温高压条件下,立方氮化硼聚晶层与碳化钨硬质合金层中的元素彼此扩散、渗透,形成金属过渡层,将二者牢固地结合在一起。     

高通量计算二维材料界面摩擦

建立了基于第一性原理方法研究二维材料界面摩擦的高通量计算程序,该程序实现了自动化批量建模、批量提交任务、多任务并发计算,以及计算结果自动收集、处理和图像绘制,使用该程序可以节省时间.采用此程序计算了不同层间距离下双层氮化硼和双层石墨烯的滑移势能面,及层间界面摩擦力和摩擦系数.研究发现,随着层间距离减小,双层氮化硼界面的平均摩擦力近似线性增大,摩擦系数为0.11—0.17,双层石墨烯界面摩擦力先增大后减小再增大,其摩擦系数在12 nN载荷下达到最小值(0.014),这些结果与已有研究结果一致,验证了该计算程序的可靠性.此外还研究了表面氢化和氟化对双层氮化硼界面摩擦的影响,发现氟化氮化硼/氮化硼界面的摩擦系数更低.     

压力对PCBN复合材料烧结性质的影响及烧结机理研究

在高温高压条件下,通过硬质合金基体的高压原位熔渗法,研究了合成压力对聚晶立方氮化硼(PCBN)复合材料致密度、颗粒形貌及耐磨性的影响。测试结果显示:随着合成压力的提高,PCBN复合材料的致密度和耐磨性均有一定程度的提升。利用扫描电子显微镜和能谱仪对碳化钨合金基体与立方氮化硼的结合界面进行了测试分析。测试结果表明:在高温高压条件下,立方氮化硼聚晶层与碳化钨硬质合金层中的元素彼此扩散、渗透,形成金属过渡层,将二者牢固地结合在一起。     

悬空氮化硼厚度依赖热导率研究

本文研究了多层悬空氮化硼的面内热导率。我们通过机械剥离法得到多层氮化硼,并利用干法转移技术将其悬空于微桥器件上,进而进行热导率测量。在干法转移的过程中,我们使用二甲基硅氧烷(PDMS)作为氮化硼载体,这样可使得样品表面残留的有机物更少,样品质量更高。本工作测得四层悬空氮化硼的面内热导率在室温达到286 Wm~(-1)K~(-1),与已知块体氮化硼的热导率大小相当,但低于双层氮化硼样品的结果,从而在实验上证明了悬空氮化硼面内热导率具有厚度效应。     

Three-dimensional polymer optical waveguide interleaver with selectable channel spacing

A polymer optical waveguide interleaver with selectable channel spacing and adjustable flat top based on a three-dimensional structure is reported. This interleaver is made of two waveguides laid in two different but parallel layers. These two waveguides cross-over each other three times, with an electrode heater deposited on top of each crossing point to adjust the coupling ratio between the guides. This 3D structure has many advantages over the corresponding 2D counterpart such as the ease of fabrication and the adjustment of coupling ratio between the two waveguides. The channel spacing can be selected to be either 1.6 nm, 0.8 nm or 0.53 nm simply by changing the power supplied to each heater; also at the 1.6 nm channel spacing it is possible to create a flat-top spectral response (0.6 dB ripple over 0.5 nm), again by tuning the coupling ratios using the electrical heaters.     

Fabrication of 550 nm gratings in fused silica by laser induced backside wet etching technique

A series of 550 nm spacing gratings were fabricated in fused silica by laser induced backside wet etching (LIBWE) method using the fourth harmonic of a Q-switched Nd:YAG laser (wavelength: λ = 266 nm; pulse duration: FWHM = 10 ns). During these experiments we used a traditional two-beam interference method: the spatially filtered laser beam was split into two parts, which were interfered at a certain incident angle (2θ = 28°) on the backside surface of the fused silica plate contacting with the liquid absorber (saturated solution of naphthalene-methyl-methacrylate c = 1.85 mol/dm³). We studied the dependence of the quality and the modulation depth of the prepared gratings on the applied laser fluence and the number of laser pulses. The surface of the etched gratings was characterized by atomic force microscope (AFM). The maximum modulation depth was found to be 180-200 nm. Our results proved that the LIBWE procedure is suitable for production of submicrometer sized structures in transparent materials.     

Funnel-Shaped Arrays of Metal Nano-Cylinders for Nano-Focusing

We analyse funnel-shaped arrays of metal nanocylinders that can be potentially used as waveguides for nanofocusing of light. The proposed structures consist of Ag nanocylinders with gradually changed radii and discrete spacing arranged like side-view funnels with different angles. Finite-difference and time-domain simulations demonstrate that the proposed structures with different spacings and funnel angles have versatile light propagation characteristics. These structures can focus the incident Gaussian light beam （200nm at its full width at half maximum） into beam sizes of 10hm, 15nm, and 20hm, respectively, which corresponds to the transmission efficiencies of 38%, 40%, and 80%.     

Gold coatings on polyethyleneterephthalate nano-patterned by F2 laser irradiation

In this work we present periodic surface structures generated by linearly polarized F₂ laser light (157 nm) on polyethyleneterephthalate (PET). Atomic force microscopy was used to study the topological changes induced by the laser irradiation. The laser irradiation induces the formation of periodic ripple structures with a width of ca 130 nm and a height of about 15 nm in the fluence range 3.80-4.70 mJ/cm² and the roughness of the polymer surface increases due to the presence of these periodic structures. Subsequently, the laser modified PET foils were coated with a 50 nm thick gold layer by sputtering. After Au deposition on the PET foils with ripple structure, the roughness of surface decreases in comparison to PET with ripples without Au coating. For 50 nm thick Au layers, the ripple structure is not directly transferred to the gold coating, but it has an obvious effect on the grain size of the coating. With considerably thinner Au layers, the ripple structures are smoothened but preserved.     

Synthesis and performance of core-shell structured ZnO/In2O3 composites in situ growth

Core-shell structured ZnO/In₂O₃ composites were successfully synthesized via situ growth method. Phase structure, morphology, microstructure and property of the products were investigated by X-ray diffraction (XRD), TG-DTA, field emission scanning electron microscopy (FESEM), energy-dispersive spectrometry (EDS), transmission electron microscope (TEM) and photoluminescence (PL). Results show that the core-shell structures consist of spindle-like ZnO with about 800 nm in length and 200 nm in diameter, and In₂O₃ particles with a diameter of 50 nm coated on the surface of ZnO uniformly. HMTA plays an important role in the formation of core-shell structures and the addition of In₂O₃ has a great effect on PL spectrum. Possible mechanism for the formation of core-shell structures is also proposed in this paper.     

Corrosion resistant surface for vanadium nitride and hafnium nitride layers as function of grain size

In this research it was studied vanadium nitride (VN) and hafnium nitride (HfN) film, which were deposited onto silicon (Si (100)) and AISI 4140 steel substrates via r.f. magnetron sputtering technique in Ar/N₂ atmosphere with purity at 99.99% for both V and Hf metallic targets. Both films were approximately 1.2±0.1 µm thick. The crystallography structures that were evaluated via X-ray diffraction analysis (XRD) showed preferential orientations in the Bragg planes VN (200) and HfN (111). The chemical compositions for both films were characterized by EDX. Atomic Force Microscopy (AFM) was used to study the morphology; the results reveal grain sizes of 78±2 nm for VN and 58±2 nm for HfN and roughness values of 4.2±0.1 nm for VN and 1.5±0.1 nm for HfN films. The electrochemical performance in VN and HfN films deposited onto steel 4140 were studied by Tafel polarization curves and impedance spectroscopy methods (EIS) under contact with sodium chloride at 3.5 wt% solution, therefore, it was found that the corrosion rate decreased about 95% in VN and 99% for HfN films in relation to uncoated 4140 steel, thus demonstrating, the protecting effect of VN and HfN films under a corrosive environment as function of morphological characteristics (grain size).     

Injection of synthesized FePt nanoparticles in hole-patterns for bit patterned media

FePt nanoparticles of uniform sizes, compositions, and crystal structures can be obtained by chemical synthesis. Additionally, the nanoparticles can be well dispersed by the adsorption of a surfactant on the nanoparticle surface. Previously, the immobilization of FePt nanoparticles on a thermal oxide Si substrate was carried out by chemical synthesis, utilizing the Pt-S bonding between the -SH functional group in (3-mercaptopropyl)trimethoxysilane, MPTMS and Pt in FePt nanoparticles. However, controlling FePt nanoparticle arrays by this synthesis method was very difficult. In the present study, we attempted to control the distortion of the arrangement of FePt nanoparticles using an MPTMS layer modified with a silane coupling reaction and a geometrical structure prepared by ultraviolet nanoimprint lithography (UV-NIL). In this study, the hole-patterns used for the geometrical structure on Si(1 0 0) were 200 nm wide, 40 nm deep, and had a 500 nm pitch. The 5.6 nm FePt nanoparticles were used to coat the hole-patterns by using a picoliter pipette. An XHR-SEM image clearly revealed that the FePt nanoparticles were successfully arranged as a single layer with an average pitch of 10.0 nm by Pt-S bonding in the hole-patterns on Si(1 0 0).     

生物质焦油制备的碳量子点

利用生物质热解副产物焦油为碳前驱体,采用反相微乳液法和P_2O_5碳化法制备了荧光碳量子点。反相微乳液法制备的碳量子点为规则球状结构,粒径均一,平均粒度为4.3 nm,晶面间距为0.25 nm,对应于石墨碳的(100)晶面;P_2O_5碳化法制备的碳量子点为蜂窝状结构,平均粒径为6.8 nm,晶面间距为0.33 nm,对应于石墨碳的(002)晶面。两种方法制备的碳量子点均具备石墨结构特征,碳量子点荧光产率分别为85.3%和24.3%。     

Role of crystal orientation on the magnetic properties of CoFe2O4 thin films grown on Si (1 0 0) and Al2O3 (0 0 0 1) substrates using pulsed laser deposition

We report the influence of crystal orientation on the magnetic properties of CoFe₂O₄ (CFO) thin films grown on single crystal Si (1 0 0) and c-cut sapphire (Al₂O₃) (0 0 0 1) substrates using pulsed laser deposition technique. The thickness was varied from 200 to 50 nm for CFO films grown on Si substrates, while it was fixed at 200 nm for CFO films grown on Al₂O₃ substrates. We observed that the 200 and 100 nm thick CFO-Si films grew in both (1 1 1) and (3 1 1) directions and displayed out-of-plane anisotropy, whereas the 50 nm thick CFO-Si film showed only an (1 1 1) orientation and an in-plane anisotropy. The 200 nm thick CFO film grown on an Al₂O₃ substrate was also found to show a complete (1 1 1) orientation and a strong in-plane anisotropy. These observations pointed to a definite relation between the crystalline orientation and the observed magnetic anisotropy in the CFO thin films.     

Broadband cascaded four-wave mixing by using a three-pump technique in optical fibers

We present an optimized technique to generate frequency combs by four-wave mixing in highly nonlinear low-dispersion fibers. Three-pumps with unequally spaced frequencies (separation of 100 and 200 GHz) were used. The pumps were located relatively close to the zero-dispersion wavelength of the fiber. Using this three-pump technique we obtained a total of 275 FWM products experimentally, with a 100 GHz spacing and a spanning of over 220 nm.